The purpose of this study was to examine the relationship between the catecholamine threshold (CAT) and the double product break point (DPBP) under various condition and to pursue the validity of DPBP as the marker of aerobic capacity for the optimal exercise intensity to improve physical fitness. We found the following.(1) Significant negative relationship between DPBP and CAT under both nomoxic and hyposic conditions was found. As expected, the slope of the DP against Vo2 increases steeply above the CAT. These findings confirmed our hypothesis that DPBP implies steeper increases in sympathetic nervous activity.(2) Hypoximia did not alter the % VO2 max at the DPBP. In addition, both the DPBP and VT in patients with heart disease changed proportionally with the VOィイD22ィエD2 peak after exercise training. These findings suggest that it is useful marker of the endurance capacity as well as to determine the optimal exercise intensity to improve aerobic capacity.(3) Beta blockers did not dissociate DPBP from VT as may have been expected. The strong positive correlation between DPBP and VT indicates that DPBP can be used as an alternative to VT.(4) The slope of the regression line formed for the mean blood pressure above the VT was greater than that below the VT. This result suggests that the mean blood pressure increases nonlinearly to increase in work load during graded exercise and the slope becomes much steeper above the VT.